Add optional dependencies

What does this MR do?

• Adds support for nitrile#25 (closed), to display when dependencies are optional.
• Adds a content-type header for API endpoints

Changes to public APIs

Dependencies in the registry (/api/packages) will have two possible formats (see nitrile!88 (merged)):

• "base": "^1.0.0", a simple version constraint (the old format)
• "base": {"version": "^1.0.0", "optional": true}, a version constraint with additional info (currently only optional) (the new format)

This is the immediate result of changes to the metadata format in Nitrile; the website itself does not generate these structures.

Author's checklist (required)

• Intermediate commits compile (use git rebase -i main if not)
• Newly added code has a style consistent with the rest of the repository
• Newly added code is documented (guidelines)

Related issues

nitrile#25 (closed)